Modern crystal growth techniques have made it possible to implement a very wide range of laser structures.
One such laser structure that has been implemented recently is referred to as a "vertical cavity laser".
Such vertical cavity lasers have given rise to literature that is very abundant.
As shown diagrammatically in accompanying FIG. 1, the essential features of vertical cavity lasers generally comprise an active recombination region 10 surrounded on either side by confinement regions 20 and 22, themselves placed between mirror-forming zones 30 and 32.
The bottom mirror-forming zone 32 is placed on a substrate 42, while the top mirror-forming zone 30 receives a contact layer 40 for injection purposes.
Proper operation of such structures relies on making good quality mirrors 30 and 32 above and below the active portion 10 of the component.
The most efficient way of making such mirrors 30 and 32 is to make a repeated stack of two transparent layers having different refractive indices, with each layer being of thickness equal to .lambda./4.
In FIG. 1, reference 50 designates the vertical upwards emission obtained through the mirror 30, and reference 52 designates the vertical downwards emission obtained through the mirror 32.
Reference may usefully be made to the document by J. L. Jewell, J. P. Harbison, A. Sheer, Y. H. Lee, and L. T. Florez in IEEE J. Quantum Electron QE-27, p. 1332-1346 (1991) for an outline description of such a laser structure corresponding to FIG. 1.
Reference may also be made to the document by M. Shimada, T. Asaka, Y. Yamasaki, H. Iwano, M. Ogura, and S. Mukai in Appl. Phys. Lett., 57, p. 1289-1291 (1990) for a more complete description of a real laser, as shown in FIG. 2.
This type of mirror formed on a stack of transparent layers is known by the term "Distributed Bragg Reflector", commonly abbreviated to DBR.
Such vertical emission laser structures comprising Bragg reflection mirrors are particularly easy to make out of materials based on GaAs, since layers of AlAs and of AlGaAs containing about 20% Al present refractive indexes that are sufficiently different and they are easy to grow epitaxially one on another.
Structures of this type have been made in numerous configurations.
Nevertheless, there remains the difficulty of the high electrical contact resistance of the reflecting layers 30 and 32.
This high contact resistance is related to band discontinuities at each of the interfaces, which discontinuities slow down the passage of charge carriers.
Various solutions have been envisaged to attempt to remedy this difficulty: for example by making transition layers in a variable period superlattice, or by incorporating layers of alternating dopants to constitute interface dipoles that limit discontinuity.
It will be observed that in the vertical emission laser structure shown in FIG. 2, the p-type contact does not pass via a Bragg mirror in order to minimize the resistance of the device.
All of these modifications do indeed provide improvements, but they do not lead to major changes since the internal electrical resistance of the device remains very high.
On this point, reference may be made in particular to the document by J. L. Jewell, J. H. Lee, A. Scherer, S. L. McCall, and N. A. Olsson in Opt. Eng., 29, p. 210-214 (1990).
In contrast, when the wavelengths of interest are for optical telecommunications, i.e. 1.3 .mu.m or 1.55 .mu.m, it is necessary to use components on a substrate 42 made of InP.
Under such circumstances, the drawbacks of the GaAs system are conserved, and there are two further difficulties. Firstly since the wavelength is longer, the layers must be made thicker.
Secondly, the differences in refractive index are smaller so it is therefore necessary to make a stack comprising a larger number of layers 32 in order to obtain a satisfactory reflection coefficient.
By way of example, W. Tsang was able to obtain a good quality DBR on InP only by using a total thickness of 11 .mu.m.
That laser is described in the publication by F. S. Choa, K. Tai, W. T. Tsang, and S. N. G. Chu, in Appl. Phys. Lett., 59, p. 2820-2822 (1991).
FIG. 3 shows the reflectivity, as measured at three different positions along a wafer, of a DBR made up of 45 successive periods of InP/InGaAsP as proposed by W. Tsang.